Re: [Xen-devel] experience with gnbd

[Ian Pratt <Ian.Pratt@xxxxxxxxxxxx>]

> What kind of performance improvement did you experience? This is not 
> just due to the NetApp filer being on a separate network with a router 
> or firewall in between (if I recall correctly)?

With gnbd we were getting sequential read performance equivalent
to native disk performance 40MB/s (though with more CPU burn).

With Linux 2.4 and linux-iscsi-3.6.1 talking to our NetApp filer
we were seeing around 10MB/s, as I recall. It's not a fair
comparison as we don't know what else was loading the filer at
the time. The NetApp probably isn't optimised for iscsi anyhow
(it's a great NFS/CIFS server).

I haven't investigated the level of CRC etc protection offered by
gnbd vs iscsi, but I doubt gnbd is so sophisticated.  It seems to
work pretty well, though, and is easy to set up.

[Just to follow up to my previous message, when building gnbd
various binaries failed to build due to not having the magma
headers/libraries installed. I just did a 'make -i' to ignore the
errors, and ended up with a working system providing you use the
'-c' option to gnbd_export. The magma stuff is to do with cluster
monitoring.]
 
> > I haven't tried it, but the csnap writeable snapshot driver looks
> > worth investigation too -- its design is rather more reassuring
> > than lvm2 snap.
> 
> Perhaps it is better to have the writable/client-specific parts of your 
> root filesystem (/tmp, /var/tmp, perhaps /etc) mounted via NFS (or 
> something else, or just as symlinks to a separate device) on top of a 
> read-only generalized rootfs (like the debian diskless packages used to 
> do), rather than trying to handle this at the block-level. It seems to 
> me all sorts of bad stuff can happen with a writable block-level 
> overlay, for instance if you try to upgrade the filesystem underneath.

If only there was a decent file system-level
CoW/overlay/union/stackable file system for linux...

There are a whole bunch of implementations, but none of them seem
particularly well supported. I don't know of any that exist for
2.6. Does anyone on the list?

We have one that works as a user-space NFS server, but "lightning
fast" is not how I'd describe it...

Ian


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